Electrosurgery has found widespread use in the medical field to perform cutting and coagulating operations. Normally, the patient is placed in contact with a patient electrode or plate connected to the patient terminal of a radio frequency (RF) source. The active terminal of the RF source is then connected to the active electrode of an electrosurgical instrument which is commonly utilized as a cutting or coagulating electrode when brought into patient contact. When so utilized, the RF source applies a high density current to the active electrode at a relatively high voltage and this causes a localized cutting or coagulating action. The current, after flowing through the active electrode, is normally returned through the patient plate to the RF source. To insure a low current density other than at the active electrode, the patient plate is designed to contact the patient over a relatively large area. This results in the needed low current density and thus prevents the occurrence of localized electrical burns as long as the patient plate contacts the patient over the large area.
If the return path connecting the patient plate to the RF source is broken, however, or if the patient should move out of contact with a large area of the patient plate, it has been found that electrical burns can result since there is no longer a low current density connection for return of the RF energy. Such a burn could occur, for example, where there is a secondary return contact to the patient since current can flow through the secondary return contact and thus cause localized burning of the patient at the point where the secondary return contacts the patient.
Such secondary return contacts could exist, for example, where monitoring electrodes are connected to the patient, where there is grounded adjacent metallic equipment, or where vertical supports are utilized for supporting ancillary equipment such as overhead lights. Since such secondary contacts with the patient are commonly in localized areas, the current density at these areas can be high and hence result in electrosurgical burns at these contact points.
Electrosurgical burns as described hereinabove can be quite severe since the patient is often unconscious during surgery and hence the existence of a condition causing such a burn could go unnoticed for a considerable length of time.
One method for minimizing the burn hazard that has been suggested is to provide an isolated output. It has been found, however, that the safety of such a circuit is limited by RF leakage currents, which depend upon output-to-ground capacitances and the RF waveform. When these factors preclude the use of an isolated output, an internal ground applied to the patient terminal becomes necessary.
While safety circuits have been suggested and/or utilized heretofore in an attempt to prevent such a condition or to at least minimize burns where such a condition comes into existence, it has been found that advantages can be obtained by utilizing a safety circuit with components different from those heretofore suggested and/or utilized. An electrosurgical safety circuit is shown, for example, in U.S. Pat. No. 3,683,923 issued Aug. 15, 1972, to Robert K. Anderson and assigned to the assignee of the present invention.